Makers Mark is a good cheapish bourbon. I'm a gin man myself, but it's not exactly a popular thing to take warm and without any mixers (although 1 lime will net you 4 gimlets). I am, however, digging the reasoning to bring rum.

/\ Was this sarcasm? Are you sure?
|| Read it again, I'll wait.
|| The results may surprise you.

If you're partial to beer and just want to avoid the coolers: buy six-packs of cans and tie them to a short rope (half a meter - just enough to be in the water). Toss the beer behind the raft/canoe and drag it behind you in the water. The river water is going to be cool, the beer will stay at the same temperature as the water. A typical fast-moving river in North America is around 10 degrees (50-ish F). A typical household refrigerator is set to about 4 degrees (40-ish F). It's not going to be ice cold, but it'll be cool enough to drink and enjoy on a hot day: no worse than a pint on the patio after 5 minutes. A couple of thick elastic bands to hold all six cans together is usually enough to keep them from breaking free in rough waters.

isopropyl alcohol is great. 99,9% ftw.

Anything you can buy at retail (or from lab supply shops) has been adulterated with various compounds to prevent drinking. In order to be sold in western nations it'll typically contain a fair bit of methanol (a poison that causes blindness and organ failure) and pyridine (which makes you puke and causes stomach pain)

I'd be going with vodka. Unless everybody is a fan of whiskey.
I myself would be taking beer over anything else.. including a first aid kit.
Vodka is pretty versatile and it can be mixed better with peoples preferences.